The most common and most likely answer to this crossword clue is the 6 letter word MAPLES
I've seen this crossword clue in The Times
M
A
P
L
E
S
More clues leading to the result MAPLES
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The straight part
We know that "Trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trees = maples.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MAPLES mean?
MAPLES noun- A tree of the Acer genus, characterised by its usually palmate leaves and winged seeds.
- The wood of such a tree, prized for its hardness and attractive appearance